High molecular weight polyesters and particularly polyesters and copolyesters of glycols and terephthalic or isophthalic acid have been available for a number of years. These are described inter alia in Whinfield et al, U.S. Pat. No. 2,465,319 and in Pengilly, U.S. Pat. No. 3,047,539. These patents disclose that the polyesters are particularly useful as film and fiber-formers.
With the development of molecular weight control, the use of nucleating agents and two-step molding cycles, poly(ethylene terephthalate) has become an important constituent of injection moldable compositions. Further, poly(1,4-butylene terephthalate), because of its very rapid crystallization from the melt, is uniquely useful as a component in such compositions. Work pieces molded from such polyester resins, in comparison with other thermoplastics, offer a high degree of surface hardness and abrasion resistance, high gloss, and lower surface friction.
Furthermore, in particular, poly(1,4-butylene terephthalate) is much simpler to use in injection molding techniques than poly(ethylene terephthalate). For example, it is possible to injection mold poly (1,4-butylene terephthalate) at lower mold temperatures of from about 450.degree. F. to 500.degree. F. to produce highly crystallized parts in short cycle times. On account of the high rate of crystallization, even at low mold temperatures, no difficulty is encountered in removing the moldings from the molds.
It is also known to add certain additives to thermoplastic polyesters to enhance or provide certain properties. For example, Rein et al, U.S. Pat. No. 3,405,198, disclose the use of polyethylene in poly(ethylene terephthalate) as an impact modifier. Holub et al, U.S. Pat. No. 4,122,061, disclose polyester compositions which comprise a poly(1,4-butylene terphthalate) resin, a poly(ethylene terephthalate) resin, a fibrous glass reinforcement, alone or in combination with a mineral filler and, as an impact modifier therefore, a polyolefin or olefin based copolymer resin including polyethylene and propylene-ethylene copolymer. Cohen et al, U.S. Pat. No. 4,185,047, disclose the use of high pressure low density polyethylene in thermoplastic polyester compositions, particularly poly(ethylene terephthalate) and poly(1,4-butylene terephthalate) for improved part releasability. All of the aforementioned patents are incorporated herein by reference.
However, it has been discovered that is difficult to render such polyester resins, modified or unmodified, flame retardant without sacrificing some of their inherent superior physical properties. Specifically, the use of conventional flame retardant additives may tend to prove detrimental to certain physical properties of the flame retardant polyester composition. One such property that may be hindered is the material processability, including the melt viscosity, of the resulting flame retardant composition. It has generally been discovered that superior processing is exhibited, for example, by tests that measure the melt viscosity of a given material, and that, as a rule, the lower the melt viscosity of a given polyester material the better is its ability to be processed in a typical commercial operation.
It has surprisingly been discovered that standard polyester flame retardant compositions, when modified with a novel flame retardant additive composition that is comprised of a halogenated polycarbonate resin, an antimony compound and a polyetherimide ester elastomer have superior processing properties than polyester compositions not modified with this novel flame retardant additive or modified with conventional flame retardant compositions. This superior processability may be shown by the lower melt viscosity exhibited by examples of the present invention over a control sample formulated with a conventional flame retardant additive. In addition, it has been discovered that various other physical properties of polyester compositions formulated with the novel concentrate of the present invention are comparable to those properties exhibited by the control samples.
In summary, it has been unexpectedly discovered that specified polyesters can be compatibly and advantageously made flame retardant with a specified flame retardant additive.